Don Krieg, also known by his epithets Foul-Play Krieg and the Pirate Fleet Admiral, is a prominent antagonist from the East Blue Saga. He is the captain of the Krieg Pirates and, at the height of his power, commanded the most formidable pirate armada in the East Blue, a fleet of fifty ships with a crew of five thousand men. His reputation was built on a series of ruthless and deceptive tactics, including posing as a Marine officer to commandeer his first vessel and his signature strategy of flying a white flag of surrender to ambush unsuspecting opponents.

Driven by an unquenchable ambition, Krieg set his sights on conquering the treacherous Grand Line after dominating the East Blue. However, his massive fleet was systematically annihilated just one week into the journey by the Warlord of the Sea, Dracule Mihawk, who destroyed all but his flagship out of sheer boredom. Krieg and his hundred surviving crew members narrowly escaped with their lives, drifting back to the East Blue on the verge of starvation. This humiliating defeat defines his role in the narrative, as he arrives at the floating restaurant Baratie, a desperate and broken man who quickly reverts to his true nature.

Krieg's personality is defined by his belief that victory is the only thing that matters, and he will employ any underhanded method to achieve it, viewing concepts like honor, mercy, and pride as sentimental weaknesses. He is arrogant, cruel, and deeply dishonest, exuding supreme confidence in his own strength and his arsenal of hidden weapons, insisting he is the most powerful man in the world even after his crushing defeat. He rules his crew through absolute fear and draconian force, demanding unquestioning obedience and punishing any hesitation or dissenting opinion with death. This cruelty is coupled with a tactical and strategic mind; Krieg is a capable strategist who understands the value of his subordinates' strength and the power of deception, using decoys and disguises, such as dressing his first mate Gin as himself to act as a decoy, to avoid unnecessary risks.

His key relationships are defined by this tyrannical leadership. His most significant bond is with his loyal right-hand man, Gin. Gin is fiercely devoted, carrying out his captain's most brutal orders without hesitation, partly out of respect but largely out of a deep-seated fear. In the Baratie Arc, after being fed by the chef Sanji, Gin pleads for mercy for the restaurant, a moment of compassion that Krieg sees as the ultimate betrayal. In response, Krieg demotes Gin and attempts to execute him with a poison gas bomb, an act that demonstrates his willingness to discard even his most trusted subordinate for the slightest sign of disobedience. Another subordinate, Pearl, shares Krieg's philosophy of foul play, but the captain considers him useful but expendable. Beyond his crew, Krieg holds a deep, furious grudge against Dracule Mihawk for destroying his fleet, though Mihawk himself views Krieg with utter contempt. He becomes an enemy of Monkey D. Luffy and the crew of the Baratie when he attempts to steal their ship and Zeff's logbook to make a second attempt on the Grand Line.

Don Krieg is a formidable fighter whose power is almost entirely derived from his customized wootz steel armor, which serves as a mobile arsenal. His physical strength is immense, allowing him to lift and throw heavy objects like the giant cannonball-firing fish used by the Baratie chefs. The armor itself is heavily reinforced, providing a strong defense that can withstand powerful blows. From within this suit, Krieg can deploy a vast array of weaponry. These include the Drill Diamond Knuckle, diamond-studded gauntlets for close-quarters combat; the Poison Gas Bomb MH5, a deadly chemical weapon that releases a lethal cloud requiring a gas mask to survive; the Flamethrower, capable of incinerating a ship's mast in seconds; the Needle Machine Gun, which fires poisoned projectiles; the Morningstar, a large spiked ball on a chain; and his primary weapon, the Battle Spear, an explosive-tipped lance that detonates on impact. Despite his arsenal and strength, he was ultimately defeated by Luffy after a prolonged and brutal battle.

Although initially defeated and carried away from Baratie by his first mate Gin, the character's story does not end there. His fate was revealed years later. Having recovered from his defeat, Don Krieg and Gin were shown to have survived and eventually made their way to the pirate island of Hachinosu, where they became affiliated with the Blackbeard Pirates. They later appear together listening to a world-shaking broadcast, confirming that the former Pirate Fleet Admiral of the East Blue continues his career as a pirate in the New World.